A technique of irradiating a banknote with lights of different wavelength bands, and forming an image relating to each of the wavelength bands based on a light intensity of light reflected from the banknote is known in the art. For example, Patent Document 1 discloses a technique in which a plurality of light emitting elements are used as light sources that emit visible lights of respectively different wavelength bands, a banknote is irradiated with the visible light while sequentially turning on/off each of the light emitting elements, a light intensity of the visible light reflected from the banknote is detected with a respective light receiving element, and images are formed in which pixel values represent the light intensities of the detected reflected lights.
However, in the method disclosed in Patent Document 1 in which a banknote is irradiated with visible light while sequentially turning on/off each of the light emitting elements, the opportunity of detecting the light intensity of the reflected light is disadvantageously decreased in comparison to a method in which turning on of one light emitting element is repeated. For example, assume a case in which light emitting elements corresponding to red light, green light, and blue light, are sequentially turned on. In this case, although it is possible to form an image of the red light, an image of the green light, and an image of the blue light, the opportunity of detection for the image of the red light is reduced to ⅓ in comparison to a case in which turning on of only the light emitting element corresponding to the red light is repeated. As a result, the resolution in the sub-scanning direction, which is a transport direction of the banknote, is decreased to ⅓.
Patent Document 2, for example, discloses a technique to prevent such degradation of the resolution. In this technique, three light emitting elements corresponding to the red light, the green light, and the blue light are turned on simultaneously, and three light receiving elements each covered with a color filter of red, green, and blue, respectively, are used. Accordingly, it is possible to prevent the decrease in the opportunity to detect the light intensity of the reflected light, and the same resolution as that obtained when the turning on of only the light emitting element of the red light is repeated can be obtained.